How to animate skinned joint translation in local space?


I have an animation of a character and I need to add animation to deform the skeleton where some joints will translate to a new position over time (think making the arms longer), but I need to move them in relation to the bind pose so that the animation still plays correctly.

Seems like I should be able to use a blendMatrix between the start and end matrix for each joint but after experimenting I cant figure out where to output the blended transform so they are moved relative to the bind pose. Thank you for any help!